This invention relates to a ground wire monitoring system for continuously checking the integrity of a grounding circuit of a machine operated by electric power and diposed at a distance from a power source. The apparatus is particularly adapted for use with an alternating current powered mining machine that is supplied by a flexible cable from a power source such as a transformer located in a power center that is remote from the machine.
Federal regulations require the use of such grounding circuits in connection with polyphase alternating current circuits that supply power to underground machines such as continuous coal mining machines. Grounding circuits conventionally include a ground conductor that extends along with the phase conductors in a flexible cable from the protective ground of a transformer at the power source to the frame of the machine. This circuit must include means for opening the circuit breaker at the power source immediately upon failure or breakage of the ground wire. This is essential to protect personnel from hazards that might arise due to failure of insulation in the machine or other fault and to prevent the machine frame from reaching a potential that could be dangerous to personnel who might be standing on the ground and touching the machine.
Ground wire monitoring circuits have been proposed in which the monitoring circuit has embodied a pilot wire that is utilized to complete a circuit with the ground wire in which a current is circulated. Failure of either the ground wire or the pilot wire is supposed to result in the opening of the circuit breaker leading to the machine. These systems present a hazard, however, for the reason that parallel paths for current to flow may exist between the machine frame through the ground to the remote power source. These incidental grounds, such as the machine frame resting on the earth, paths through pipe lines, mine track and conveyor frames and the like also help keep the machine frame at earth potential. But the resistance of these incidental grounds are variable quantities which, at the time of an electrical fault, may not be low enough to keep the machine frame from rising to a dangerous voltage above earth. Furthermore, the presence of the incidental parallel ground paths may prevent the usual ground wire monitoring circuit from opening the circuit breaker leading to the machine in the event that the ground wire is broken but the pilot wire remains intact.
Known pilot wire systems heretofore used, such as the system shown in U.S. Pat. No. 3,335,324, cannot distinguish between a current circulating through the pilot wire and the ground wire and a current circulating through the pilot wire and parallel ground paths between the machine frame and the power source, and thus may fail to operate because of the presence of the parallel paths even though the ground wire may be broken and even though the resistance of the parallel paths may not be low enough to maintain the voltage of the machine frame at a safe level above earth in the event of a serious fault. Furthermore, with prior types of devices the usual practice for checking correct operation of the monitor circuits is by temporarily opening the ground wire. If the relay opens when the ground wire is opened, this is an indication that the relay is operating correctly; but if the relay fails to open, this could mean either that the relay is not functioning correctly (it could be stuck or jammed) or that it is being held closed by current through low resistance parallel paths. The condition of the relay can be determined by temporarily opening the pilot wire. If the relay then drops open it is functioning correctly, but if low resistance parallel paths do exist there is no way, with these prior systems, to make sure that the ground wire is unbroken.